This invention relates to internal combustion engines of the kind which use, as their source of power, a gaseous fuel stored as a gas. Many such engines have been proposed and used, in both automobile and stationary applications. It has however been common practice in such applications to prepare the charge of air and fuel gas, necessary for each power stroke of the combustion cycle, by mixing the two gaseous constituents in controllable proportions in a manifold upstream of the cylinder, in a manner and at pressures analogous to those in which air and vapourised fuel are premixed in a conventional gasoline-fuelled internal combustion engine. The gas fraction of the mixture thus displaces the equivalent volume of air, so preventing the engine from developing the best power or, if desired, fuel economy. Also, because the fuel gas must be stored at considerable pressure, typically of the order of 250 bar in order to maximise the time interval between refuellings, means must be provided between the tank and the manifold to reduce the pressure of the gas considerably, say to about 70 bar,